TNF-α (tumor necrosis factor-alpha) is a pleiotropic cytokine produced by activated macrophages/monocytes and lymphocytes which often promotes inflammatory responses leading to a variety of diseases. TNF-α is released from macrophages, monocytes and natural killer cells and play an important role in inflammatory and immune responses, including the recruitment of leukocytes to injured tissues during bacterial and other microbial infections, and following stimulation with inflammatory substances. When present in excessive quantities, TNF-α is known to cause tissue injury, and has been implicated in the pathology associated with inflammatory and autoimmune diseases.
TNF-α mediates biological effects through two distinct membrane-protein receptors, TNF-RI and TNF-RII, which differ in sequence and molecular mass. TNF-RI is reported to be present at low levels in most, if not all, human cell types, and expression of the TNF-RI gene in humans can be upregulated by infection, interferons, and modulators of second messengers, such as phorbol esters. The extracellular portions of both TNF receptors also exist in soluble forms, which are derived from membrane-bound forms of the receptors by proteolytic cleavage at the cell surface. The soluble TNF receptors retain the ability to bind TNF-α in solution. Soluble TNF receptors have been identified in urine and sera from healthy individuals, and have been shown to be elevated in some chronic diseases and following inoculation with agents that induce TNF-α.